The present disclosure relates generally to control methods and apparatuses for light dimming, and more specifically to backlight controllers and dimming control methods capable of avoiding flickering.
As superior in power-to-light conversion efficiency, product size and device lifespan, light emitting diodes (LEDs) have long being utilized in applications of lighting and backlights. For example, nowadays, the backlight modules, which were used to employ cold cathode fluorescent lamps (CCFLs) as light sources, are commonly employing LED modules.
Circuitry for driving LEDs in a backlight module normally has two stages. The first one is a power converter, which could be a switching mode power supply, for converting and providing electric energy to the LEDs so they could emits light. The second stage is a constant current controller for regulating the amplitude of the current through the LEDs.
Dimming is a common function required for a LED module, as the brightness of a backlight frequently needs adjustment. It is well known in the art that dimming methods are categorized into two ways. One is called PWM dimming or digital dimming, the other analog dimming. PWM dimming uses a digital dimming signal that determines a duty cycle of a LED, or the ratio of the time duration when the LED emits light to the cycle time of the digital dimming signal. Normally, for PWM dimming, when an LED emits light, its light intensity is a constant irrelevant to the duty cycle. Analog dimming uses an analog dimming signal instead to regulate the amplitude of the current flowing through a LED, so it emits light constantly and continuously and its brightness is determined by the analog dimming signal.
A power converter for driving a LED usually utilizes a close loop to regulate an output voltage. This close loop certainly has it limited bandwidth and response time. If the close loop is activated for a duration less than its response time, the power converter, as not having enough time to stabilize the close loop, might provide power energy not as sufficient as that for driving the LED properly. Probably, human eyes might conceive that the LED becomes dark for a while every certain period of time. This phenomenon, also called flickering in the art, is very unpleasant to human eyes and should be avoided.